High-energy (0.7 to 1MeV) electron irradiation, of samples which had been grown onto sapphire, produced shallow donors and deep or shallow acceptors at equal rates (1/cm). These data, in conjunction with theoretical considerations, were consistent only with the assumption that the shallow donor was the N vacancy, while the acceptor was the N interstitial. The N-vacancy donor energy was 0.064eV. This was much larger than the value (0.018eV) for the residual donor (probably Si) in this material. Hall-effect measurements also revealed a degenerate n-type layer at the GaN/sapphire interface. This had to be accounted for in order to determine the correct donor activation energy.

D.C.Look, D.C.Reynolds, J.W.Hemsky, J.R.Sizelove, R.L.Jones, R.J.Molnar: Physical Review Letters, 1997, 79[12], 2273-6